1. Field of the Invention
The present invention relates to infrared sensors and to methods of manufacturing the same. In particular, the present invention relates to a thermoelectric infrared sensor having a diaphragm structure which is prepared by etching a sacrificial layer on a semiconductor substrate or under a thin film and to a method for making the same.
2. Description of the Related Art
FIGS. 1A and 1B are a plan view and a cross-sectional view, respectively, of a typical conventional thermoelectric infrared sensor 1. In the thermoelectric infrared sensor 1, a thermal insulating thin film 4 is provided on a heat sink frame 2 and a cavity section 3, and two types of metals or semiconductors 5 and 6 are alternately wired on the central portion of the thermal insulating thin film 4 to form a thermopile 9 comprising thermocouples connected in series. These metals or semiconductors 5 and 6 are connected at portions above the heat sink frame 2 to form cold junctions 7 of the thermocouples, and are also connected at portions above the cavity section 3 to form hot junctions 8 of the thermocouples. The cold and hot junctions are formed on the thermal insulating thin film 4. The thermopile 9 has external electrodes 11 at both ends. The hot junctions 8 are covered with an infrared-absorbing layer 10.
Infrared rays incident on the thermoelectric infrared sensor 1 are absorbed in the infrared-absorbing layer 10 to generate heat which is conducted to the hot junctions 8. Thus, a temperature difference is generated between the cold junctions 7 and hot junctions 8 formed above the heat sink 2, which produces an electromotive force between the external electrodes 11 of the thermopile 9. Suppose that the thermoelectromotive force generated at a junction (or a thermocouple) of two metals or semiconductor elements 5 or 6 at a temperature of T is represented by "PHgr"(T), and the number of the hot junctions 8 and the cold junctions 7 is m, respectively. When the temperature at the hot junctions 8 is TW and the temperature at the cold junctions 7 is TC, the electromotive force V generated between the external electrodes 11 of the thermopile 9 is represented by equation (1):
V=m["PHgr"(TW)xe2x88x92"PHgr"(TC)]xe2x80x83xe2x80x83(1)
When the temperature TC at the heat sink frame 2 is known, the temperature TW at the hot junctions 8 is determined from the electromotive force V generated between the external electrodes 11. Since the temperature of the infrared-absorbing layer 10 increases according to the dose of the infrared rays which are incident on the infrared sensor 1 and are absorbed in the infrared-absorbing layer 10, the dose of the infrared rays incident on the infrared sensor 1 can be determined by measuring the temperature TW at the hot junctions 8.
In general, in such an infrared sensor 1, the heat sink frame 2 is comprised of a silicon substrate and the heat insulating film 4 is composed of SiO2 film having a low thermal conductivity. The SiO2 film, however, has high compressive stress. When the heat insulating film 4 is formed of a single SiO2 layer, the heat insulating film 4 may break in some cases.
Thus, in another conventional infrared sensor 12 shown in FIG. 2, a heat insulating film 4 on a silicon heat sink frame 2 comprises a Si3N4 layer 13, a SiO2 layer 14, and a Si3N4 layer 15, a thermopile 9 is covered with a protective film 16, and an infrared-absorbing layer 10 is provided thereon. In this configuration, the Si3N4 layers 13 and 15 have tensile stress and the SiO2 layer 14 has compressive stress. Thus, the stress of the heat insulating film 4 formed by laminating these layers is relaxed to avoid damage to the heat insulating film 4.
Since the Si3N4 layers 13 and 15 are formed by a low pressure CVD (LPCVD) process, the heat insulating film 4 composed of the Si3N4 layers 13 and 15 and the SiO2 layer 14 is produced at high facility and production costs. As a result, the infrared sensor 12 is inevitably expensive.
In another infrared sensor 17 shown in FIG. 3, a heat insulating film 4 on a heat sink frame 2 is a multilayered film composed of SiO2 layers and Al2O3 layers which are formed by an ion plating process. Also, in such a configuration, the tensile stress of the Al2O3 layers offsets the compressive stress of the SiO2 layers to avoid damage to the heat insulating film 4.
Since the Al2O3 films have a high thermal conductivity, the heat generated by the infrared rays in an infrared-absorbing layer 10 dissipates to the heat sink frame 2 via the Al2O3 layers. Thus, an increase in the temperature at the hot junctions is suppressed. Accordingly, the sensitivity of the infrared sensor 17 is reduced.
Accordingly, it is an object of the present invention to provide an infrared sensor which can be produced at reduced production costs and which exhibits high sensitivity.
It is another object of the present invention to provide a method for manufacturing the infrared sensor.
According to an aspect of the present invention, an infrared sensor comprises a heat insulating thin-film, a heat sink section for supporting the heat insulating thin-film, and a thermoelectric infrared detecting element provided on the heat insulating thin-film, wherein the heat insulating thin-film comprises an insulating layer primarily composed of aluminum oxide having partial oxygen defects and a silicon oxide layer. The thermoelectric infrared detecting element converts thermal energy into electrical energy. Examples of such elements include thermopiles (thermocouples), pyroelectric elements, and bolometers.
Since the insulating layer primarily composed of aluminum oxide having partial oxygen defects exhibits tensile stress and a low thermal conductivity, the aluminum oxide insulating layer offsets the compressive stress of the silicon oxide layer which is another constituent of the heat insulating thin-film. Thus, the heat insulating thin-film exhibits a low thermal conductivity and is barely damaged. Accordingly, this infrared sensor has high mechanical strength and high sensitivity. The aluminum oxide having partial oxygen defects can be readily formed by a vacuum deposition process at reduced facility and production costs.
In this infrared sensor, the aluminum oxide having partial oxygen defects is preferably represented by equation (2):
Al2O3xe2x88x92Xxe2x80x83xe2x80x83(2)
wherein the subscript X indicates the rate of the oxygen defects and is within a range of 0.05xe2x89xa6Xxe2x89xa60.5.
When X is outside of this range, the thermal conductivity of the aluminum oxide insulating layer increases.
According to another aspect of the present invention, an infrared sensor comprises a heat insulating thin-film, a heat sink section for supporting the heat insulating thin-film, and a thermoelectric infrared detecting element provided on the heat insulating thin-film, wherein the heat insulating thin-film comprises an insulating layer primarily composed of amorphous aluminum oxide and a silicon oxide layer.
Since the insulating layer primarily composed of amorphous aluminum oxide exhibits tensile stress and a low thermal conductivity, the amorphous aluminum oxide insulating layer offsets the compressive stress of the silicon oxide layer which is another constituent of the heat insulating thin-film. Thus, the heat insulating thin-film exhibits a low thermal conductivity and is barely damaged. Accordingly, this infrared sensor has high mechanical strength and high sensitivity. The amorphous aluminum oxide can be readily formed by a vacuum deposition process at reduced facility and production costs.
According to another aspect of the present invention, a method for making an infrared sensor comprises supporting a heat insulating thin-film comprising a silicon oxide layer and an aluminum oxide layer with a heat sink section, and providing a thermoelectric infrared detecting element on the heat insulating thin-film, wherein the aluminum oxide layer is formed by an electron beam evaporation process at a deposition rate of 0.8 nm/s or less.
By an electron beam evaporation process at a deposition rate of 0.8 nm/s or less, an aluminum oxide layer having partial oxygen defects or an amorphous aluminum oxide layer can be formed.